. The goals of the proposed research are to determine the diversity of Pneumocystis carinii that infect humans and various animal species by defining sequence variations in the genomes of P. carinii and to study the epidemiology of P. carinii infection. Studies conducted in the investigator's laboratory have revealed that isolates vary in nucleotide sequence in the internal transcribed spacers (ITS) of the nuclear rRNA genes and that this variation can be used to identify isolates of P. carinii that infect humans. Based on the ITS sequence variations identified to date, P.carinii isolates from humans can theoretically be classified into 6 different types of which 4 have been found to date. Preliminary epidemiological studies have revealed differences in the geographic distributions of these types. Some specimens had more than one type of ITS sequence. Experiments will be performed to determine whether 2 types of ITS sequences actually represent 2 types of organisms. Another aim will be to search for additional genomic loci that have nucleotide sequence variations useful for typing P. carinii strains. These additional typing methods may enable subclassification of each P. carinii type. Methods will be developed to type P.carinii that infect animal species other than humans since preliminary results have suggested that some P.carinii may infect both animals and humans. Epidemiological studies will be conducted to determine whether specific types of P.carinii are associated with certain underlying diseases, response to therapy, or clinical manifestations such as dissemination. Epidemiologic studies will further determine geographical distribution of each type and may allow detection of virulence variation among types of P. carinii. Typing will allow comparison of isolates from initial and second infections to determine whether infections are relapses or reinfections. The investigator has also shown that P. carinii organisms can be detected in the environmental air, supporting the hypothesis that P.carinii infections may be transmitted by the airborne. The relative importance of P. carinii infections developed by the reactivation of latent organisms that reside in lungs and by acquiring exogenous organisms will be determined. At the conclusion of the proposed study, effective molecular biology tools needed to study the epidemiology of P.carinii will have been established, and epidemiological studies will have defined the diversity of P. carinii strains and will have correlated sequence variation with particular clinical characteristics of P.carinii infection.